Single beam laser printers can print a single line to paper during one pass of the laser. In order to increase the print speed of a single beam laser printer, its internal elements can be run faster and/or at a higher clock rate. There are limits, however, to the speed at which the internal elements of a printer may run. Dual-beam laser printers overcome some of these limitations by scanning out two lines of pixel data simultaneously to a photosensitive drum.
Dual beam laser printers may be designed so that odd lines of an image are scanned out with a first laser and even lines of the image are scanned out with a second laser. For dual beam laser printers, each line of pixel data can be separately accessed in memory, and the systems often use separate Direct Memory Access (DMA) channels to main memory for the odd and even lines of the image.
In some laser printers, a top of data (TOD) event signal or top of page event signal may be sent to the image electronics of a printer when paper is fed through the laser printer. A beam detect (BD) event signal may be generated during each horizontal pass of the laser. The TOD signal may be asynchronous with the BD signal. Therefore, it is possible for the data sent to the TOD signal to be nearly a full cycle out of sync with the start of each pass, corresponding to the “hsync” or BD signal. In the case of a single-beam laser printer, this means that the data sent to the image may have a variation of up to one printed line with respect to the TOD signal, and therefore, there may be a variation in where the first line of the page will be printed from one printed page to another. For a dual-beam laser printer, the problem may be exacerbated. Since data for up to two lines of an image may be printed simultaneously, a variation of nearly a full cycle may result in a variation in the printed image of up to two lines. In general, as the number of lines drawn per cycle increases, the misalignment of the printed page with the top of page signal is exacerbated further. For example, in a tri-beam printer, three lines of data are printed simultaneously, and the difference in alignment between the TOD signal and the BD signal may result in a difference of up to three printed lines between two printed pages.
In laser printers that have multiple components, such as four-color cyan (C), magenta (M), yellow (Y), and black (K) (“CMYK”) printers, the complete image may be made up of the four components, and the four components may be laid down sequentially, one on top of the other. Image quality is based at least on the vertical alignment of the components. In a single beam printer, each of these signals may be up to one line out of alignment with each other because TOD signal is asynchronous with BD signal. In a multi-beam printer, as noted above, the potential for misalignment is exacerbated in proportion to the number of lines that are printed simultaneously. In the case of a multi-pass, multi-beam printer, because each of the components is laid down sequentially, each component of the image may be misaligned in proportion to the number of beams in the printer. The misalignment among the components contributes to image-quality reduction.
Both single and multi-beam printers may store pixel data in memories. Each of these memories may be used to store and write out a single line of pixel data. Data written into and read from these memories is often synchronized with the printing of images. Because a new line of pixel data is often stored at the same time that an old line of pixel data is written out printers often use multiple memories, each capable of storing a single line of pixel data. For multi-beam printers, the number of memories may be further increased by the number of beams in the printer. For example, a dual-beam printer may need four memories, each capable of storing a complete line of pixel data. The number of memories and accompanying circuitry or software to manage and synchronize their operation increases the cost and complexity of printers.
Thus, there is a need for a method, system, and apparatus for processing pixel data for a printer that allows alignment of the printed image and optimizes memory utilization.